In recent years, enhancement in functionality of electronic apparatuses requires an extremely high wiring density. Wiring boards with high wiring density as described above include a wiring board (hereinafter referred to as multi-wire wiring board) using insulation-coated wires as necessary wires.
As disclosed in Patent Literature 1, a multi-wire wiring board is manufactured by forming an adhesive sheet (hereinafter also referred to as “wire-laying adhesive sheet”) having adhesiveness to insulation-coated wires on an internal layer substrate on which circuits such as a power source layer and a ground layer are formed, thereafter laying wires insulation-coated with polyimide resin or the like by a numerically controlled wire-laying apparatus, fixing the wires by layering or the like, providing through holes running across the wires, and subjecting inside of each of the through holes to electroless metal plating. The term “wire-laying” means laying an insulation-coated wire on an adhesive layer, and simultaneously bonding the wire with ultrasonic waves. The insulation-coated wire used for the multi-wire wiring board includes a wire (hereinafter also referred to as “core wire”), a wire coating layer disposed around the wire, and a wire adhesive layer disposed around the wire coating layer (Patent Literatures 1 to 4).